The immediate-type hypersensitivity, such as extrinsic asthma, hay fever, and allergic responses to certain foods or drugs, is mediated primarily by one isotype of the immunoglobulins, IgE. In an IgE-mediated allergic response, the allergen binds to IgE which are bound to receptors on the surface of mast cells and basophilic leukocytes (basophils). The binding of the allergen causes crosslinking of the surface IgE molecules and hence the underlying receptors for the Fc portion of IgE (Fc.epsilon.R), thereby triggering the release of pharmacologic mediators such as histamine, the slow-reacting substance of anaphylaxis (SRA), and serotonin. The release of these mast cell and basophil products causes the pathological reactions and symptoms of allergy.
IgE is produced by a particular class of B cells, the surface IgE-bearing B lymphocytes. In individuals sensitized to specific allergens, the allergen-specific IgE is produced by these B cells continuously. Recently, Whitaker (U.S. Pat. No. 4,714,759) described a method of treating allergy patients with toxin-conjugated antibodies that were specific for IgE. The intended effect of the immunotoxin is to kill IgE-producing B cells.
In the first related U.S. patent application Ser. No. 07/140,036, filed Dec. 31, 1987, and in its continuation-in-part application Ser. No. 07/140,036, filed Jul. 29, 1988, it is noted that IgE binds to the FC.epsilon.R receptors of IgE on the surface of basophils and mast cells very strongly. The association constant, Ka, is about 1.times.10.sup.10 liter/mole. Even though IgE is not synthesized by basophils and mast cells, the very strong and stable association of IgE with Fc.epsilon.R means that IgE is virtually always present and exposed on the surface of these cells. The related applications indicate that an immunotherapeutic agent targeting the IgE on B cells must not react with the IgE on basophils and mast cells, in order to avoid triggering an allergic reaction. Antibodies which react with IgE will cross-link IgE and the underlying FC.epsilon.R on basophils and mast cells and, when administered in vivo, will induce systemic histamine release leading to anaphylaxis.
In the related patent applications the development of monoclonal antibodies that recognize an antigenic epitope present on the IgE which is expressed on B cells, but not the IgE on basophils and mast cells, is described. In addition, the method of using these antibodies or toxin-conjugates of these antibodies for treating allergy is described. The unconjugated antibody, by causing antibody-dependent cellular cytotoxicity (ADCC) and complement-mediated cytolysis and the toxin-conjugated antibodies can directly cause cytolysis of the IgE-expressing B cells. Both of these mechanisms can lead to the depletion of IgE-producing B cells without causing cross-linking of the IgE bound on the basophils and mast cells. The IgE epitopes present on B cells but absent on basophils and mast cells are termed ige.bl epitopes (bl denoting B lymphocytes).
Immunoglobulins consist of two peptide chains, a heavy chain and a light chain. In IgE, the heavy chain is designated as the .epsilon. chain. The parent application, U.S. application Ser. No. 07/468,766, describes that the IgE, and other immunoglobulins, which are bound to B cells are different from secreted IgE and other secreted immunoglobulins in that the bound form includes an extra peptide segment on the heavy chain which anchors the immunoglobulin to the B cell membrane. These peptide segments span through the B cell plasma membrane. The known DNA sequences of the gene segments encoding the cell-bound forms of immunoglobulins, including mouse and rat IgE, and in human IgM and IgD, indicate that the transmembrane anchor peptide includes three distinct portions: a middle hydrophobic portion which is embedded in the cell membrane lipid bilayer, a C-terminal hydrophilic portion located on the cytoplasmic side of the membrane, and an N-terminal portion located on the extracellular side of the membrane.
The parent application also describes the nucleotide sequence of the genomic DNA segments encoding the transmembrane anchor peptide of human .epsilon. chain. Two exons encoding the transmembrane anchor peptide of the human .epsilon. chain were identified. The first membrane exon (.epsilon.m1 exon), having 122 bp, is located about 1.5K bp from the exon for the CH4 domain. See FIGS. 1 and 2A. The second membrane exon (.epsilon.m2 exon), having 84 bp including the termination codon TGA, is separated from the .epsilon.m1 exon by 82 bp. See FIGS. 1 and 2A.
This evidence discussed in detail in the parent application indicates that a 15 amino acid peptide segment toward the N-terminus of the transmembrane anchor peptide in human .epsilon. chain, designated as the migis-.epsilon. peptide, is the extracellular portion of the human .epsilon. transmembrane anchor peptide. Because it is normally located outside the B cell, the migis-.epsilon. peptide can form entirely or in-part an epitope and antibodies which are specific to this epitope can be generated. These antibodies can be used for causing the lysis of B cells expressing membrane-bound IgE.